Many studies have examined short-term associations between ambient air pollutant concentrations and respiratory morbidity in children, but few have focused explicitly on respiratory disease in the first few years of life when susceptibilities may be greater than in later childhood or adulthood. The proposed study examines relationships between ambient air pollutant concentrations and emergency department visits for specific respiratory conditions in children ages 0-4 years. This epidemiologic investigation leverages an existing database of 12 million emergency department visits collected from 41 Atlanta metropolitan area hospitals between January 1, 1993 and December 31, 2004 as part of previous NIEHS- and EPA-funded projects. The proposed project also takes advantage of daily measurements of speciated particulate matter less than or equal to 2.5 microns in diameter (PM2.5) to complement the measurements of ambient particulate matter and gaseous pollutants available from USEPA monitors. The availability of daily speciated PM2.5 measurements is a key strength of the proposed study because the toxicity of particulate matter likely varies by chemical composition. These existing, high-quality resources will be used to investigate short-term fluctuations in ambient air pollutant concentrations in relation to several different respiratory outcomes in infants and young children, including asthma, wheeze, bronchiolitis, bronchitis, pneumonia, and upper respiratory infection. By focusing on individuals under 5 years of age, results will advance our understanding of the age-specific susceptibilities of young children to the respiratory effects of air pollution. Given the ubiquitous nature of air pollution exposure and the high prevalence of the pediatric respiratory conditions, even a modest increase in risk observed will have important public health implications. PUBLIC HEALTH RELEVANCE: Young children may have age-specific vulnerabilities to the respiratory effects of outdoor air pollution because of their immature lungs and immune systems, outdoor activity patterns, and higher ventilation rate relative to adults. We focus on this understudied age group to describe relationships between ambient air pollutant concentrations and emergency department visits for asthma, wheeze, bronchitis, bronchiolitis, pneumonia and upper respiratory infections in children less than 5 years of age.